Empty-load brake equipment is brake equipment in which the braking effort on an individual car is varied depending upon whether the car is loaded or empty. In an automatic air brake system, the brake operating motor receives air from the auxiliary reservoir under the control of a triple valve. The pressure developed in the motor depends on the amount of air delivered and the volume of the motor. It has been proposed to change the braking force by providing a volume reservoir which is arranged either to be charged by a part of the air flowing to the brake controlling motor or to be cut out of the system and vented. When this volume is connected to be charged along with the brake controlling motor, the pressure which can be developed in the motor is less than when the reservoir is cut out.
The amount of air delivered to the brake controlling motor depends upon the magnitude of the reduction of the pressure in the brake pipe up to the point at which the pressures in the reservoir and the motor have equalized. Once equalization has occurred, further reduction of brake pipe pressure will have no effect on the pressure in the brake controlling motor unless that further reduction of brake pipe pressure occurs at a rate such that an emergency application is produced.
Since the inclusion of a volumen reservoir in the charging connection of the brake operating motor increases the total volume into which the air from the reservoir may expand, it will be realized that the equalization pressure in the empty setting (volume included) will be lower than that in the load setting (volume cut out). As a result, a brake pipe pressure reduction effective to cause equalization in the load setting will not cause equalization in the empty setting. Thus, a further reduction of the brake pipe pressure will cause the pressure in the braking motors on cars in the empty setting to be increased. Commonly the engineer, when he desires a full service application of the brakes, will insure a full service application by making an over reduction. The effect of this is a loss of a proper correlation of the braking forces if empty-load brakes are in use and different settings are in effect on the cars.
According to the teachings of the U.S. McClure Pat. No. 2,068,342, dated Jan. 19, 1937, this undesirable feature can be eliminated by connecting the volume reservoir to the brake operating motor through a proportioning valve when the equipment is in the empty setting. The size of this volume reservoir is so selected that the pressure therein will equalize with the auxiliary reservoir at the same pressure which is characteristic of the brake in the load setting. In the empty setting, the brake cylinder pressure is a proportional part of the pressure in the volume reservoir as determined by the proportioning valve.
In the empty setting of his equipment, the proportioning valve of the McClure patent modifies the brake cylinder pressure throughout the time during which the volume reservoir receives fluid under pressure from the auxiliarly reservoir. In order to insure complete dissipation of the braking pressure during a brake release, McClure provided his proportioning valve with a return spring. This return spring necessarily delays the initial development of pressure in the brake cylinder. This is undesirable because the initial development of brake cylinder pressure should occur uniformly throughout the train. Control valves of the type in use in railroad freight service include a quick service feature by which an initial breaking pressure of about 10 p.s.i. is developed. Such a pressure is adequate to move the brake shoes into initial contact with the wheels.